Increased GCC optimization level.

src/Makefile

@@ -62,7 +62,7 @@ else
   endif
 endif
 
-CFLAGS += -MMD -MP -O -I common \
+CFLAGS += -MMD -MP -O3 -I common \
           -Wall -Wextra -Wno-char-subscripts $(USER_CFLAGS) \
           -DCA65_INC=$(CA65_INC) -DCC65_INC=$(CC65_INC) -DCL65_TGT=$(CL65_TGT) \
           -DLD65_LIB=$(LD65_LIB) -DLD65_OBJ=$(LD65_OBJ) -DLD65_CFG=$(LD65_CFG) \